This invention relates to a PCM (pulse code modulation) decoder for voice signals used in telephony, and more particularly to a PCM decoder whose circuit is well adapted for an LSI configuration.
With the recent advance in semiconductor technology, a circuit element fabricated in an IC or LSI configuration, has come to possess high precision and high performance and therefore further development of LSI to be used in telecommunications has been greatly demanded to decrease cost and to improve performance in the field concerned. Typical examples are an encoder for converting an analog voice signal to a PCM signal and a decoder for performing the opposite conversion. If these devices are miniaturized by LSI techniques and therefore become less expensive, a so-called single channel coding system in which both the coding and decoding functions are incorporated in every voice channel will become more practical. Hence, the design of a communication system may exhibit a high versatility.
In the case where a PCM encoder for voice transmission by telephone is used to transmit signals such as dial pulses or monitor pulses, so-called A and B signals are inserted in the eighth bit positions of the eight-bit data in signalling frames allotted to the sixth and the twelfth frames in the standard data format including such two signalling frames and the other ten non-signalling or information frames. Accordingly, a PCM decoder for telephone voice must be provided with a special decoding function, e.g. signalling function, or, what is called, bit stealing for compensating the degradation of signal-to noise power ratio (S/N) due to the reduction in number of voice data bits when the data is derived from the signalling frame.